Medical implant materials, in particular orthopedic implant materials, must combine high strength, corrosion resistance and tissue compatibility. The longevity of the implant is of prime importance especially if the recipient of the implant is relatively young because it is desirable that the implant function for the entire lifetime of a patient. Because certain metal alloys have the required mechanical strength and biocompatibility, they are ideal candidates for the fabrication of prostheses. These alloys include 316L stainless steel, chrome-cobalt-molybdenum alloys (CoCr), titanium alloys and more recently zirconium alloys which have proven to be the most suitable materials for the fabrication of load-bearing and non-load bearing prostheses.
In a traditional hip implant, CoCr alloy femoral head articulates against polyethylene. Owing to the relative hardness of each of the articulating components, this type of implant is frequently referred to in the art as a hard-on-soft implant; the CoCr alloy being the hard component of the articulating couple, and the polyethylene being the corresponding soft component. During the life of the implant, wear of the polyethylene exceeds that of the hard component. The polyethylene wear debris in turn may lead to osteolysis and loosening of the implant. A considerable amount of effort has been expended in the prior art in an effort to reduce this wear. One of the approaches has been to change the characteristics of polyethylene to improve its wear characteristics, for example by cross-linking. Another approach has been to change the characteristics of the femoral head. One such approach has been taught by Davidson (U.S. Pat. No. 5,037,438). Davidson recommends use of an oxidized zirconium surface to reduce the wear of polyethylene. Even though such approaches have led to significant reduction in wear of polyethylene, there has been a growing demand for much more wear resistant implants. This need comes from young and active patients who want to return to their normal lives after the joint replacement. Another requirement of these young and active patients is the joint stability. Typically, a larger-anatomical joint is more stable than a smaller joint. Polyethylene due to its lower strength can not be made beyond certain sizes and thus limits its use for young and active patients. This has led to emergence of hard-on-hard metal implants.
Currently, there are two primary types of hard-on-hard hip implants that are available commercially. These are metal-on-metal (including metal alloy-on-metal alloy) and ceramic-on-ceramic. The current standard material of metal-on-metal implants is high carbon CoCr alloy. The major concern with the metal-on-metal implant is the metal ion release from the joint and its unknown effects on the physiology of the human body. The advantage of metal-on-metal implants is that they can be used in larger sizes. The larger size of the implant allows greater range of motion and can provide more joint stability. The metal-on-metal implants have also been shown to be useful for resurfacing type of application where conservation of bone is desired. In such larger joints, the conventional or cross-linked polyethylene is not preferred and metal-on-metal may be the only choice available. The larger size requires polyethylene liner to be thinner. A thinner liner may not be mechanically strong, may creep more or may lead to increased wear and osteolysis and eventually failure of the implant. In general, the class of hard-on-hard implants can be significantly broadened. It can include components articulating against each other that are made from metals or ceramics or any combinations thereof. In this disclosure, the term hard refers to metals and or ceramics. Thus “hard-on-hard” can be metal-on-metal, metal-on-ceramic, and ceramic-on-ceramic. In the foregoing context, “metal” includes both pure metals and metal alloys.
The other commonly used hard-on-hard joint is ceramic-on-ceramic. The current standard material of ceramic-on-ceramic implants is alumina. Metal ion release is typically not a concern for these implants. But due to limited toughness and brittle nature of ceramics, it is difficult to make these implants in larger sizes. The ceramic components have finite probability of fracture thus leading to a potential joint failure and complications associated with the fracture of a joint.
It has been an object of much of the prior art to reduce the metal ion release and minimize the fracture risk by combining metal and ceramic components. Fisher at al. (U.S. Patent Application 2005/0033442) and Khandkar et al. (U.S. Pat. No. 6,881,229) teach the use of an implant having a metal-on-ceramic articulation. Fisher et al teach that the difference in hardness between the metallic component and the ceramic component to be at least 4000 MPa. Khandkar et. al. specifically teach use of silicon nitride ceramic components for articulating against the metallic component. In both instances, the objective is to lower the wear of mating couples. But in both instances, the fracture risk of ceramic is still significant. In both instances, the strength of ceramic component influences how large the joint size can be made. It should be noted that in both instances it is the ceramic surface that is mating with a metallic surface. As discussed in the details below, the object of this invention is to reduce the wear of the mating couples when both component surfaces are metallic in nature. In another approach, Lippincott and Medley (U.S. Pat. No. 6,059,830) teach applying geometrical constraints to the mating hip components. The '830 patent teaches the use of components such that the radius difference of the mating components is less than 50 microns. This small difference in radius will promote thicker fluid film formation and thus reduced wear of mating metallic components. The disadvantage of this method is that a sophisticated manufacturing set-up is required to produce components with such tight tolerances.
The problems relating to a low wear hard-on-hard metallic couple is not unique to the medical implant field, and exists in other fields of art as well, examples of which include automotive and aerospace components. Other bearing applications would also benefit from an improved couple.
The inventors of the present invention have found that such a demanding manufacturing approach is not necessary because significant improvements in wear reduction can be realized through the differential treatment of one surface of two contacting surfaces, and in particular where the contacting surfaces articulate against one another.